1. Field of the Invention
The present invention relates to a nitride-based semiconductor device and a method of manufacturing the same, and, in particular, to contacts for an electrode section.
2. Description of the Related Art
In recent years, semiconductor devices which uses nitride crystals such as gallium nitride (GaN) have been developed for application in light emitting elements such as blue LEDs.
FIG. 6 is a flowchart showing a conventional manufacturing processes for a nitride-based semiconductor device. First, an n type GaN layer is formed on a sapphire substrate (step S101) and then, a GaN-based emissive layer is formed over the n type GaN layer (step S102). As the GaN-based emissive layer, for example, an InGaN layer is used. Next, a p type GaN layer is formed over the GaN-based emissive layer (step S103). The GaN layers and emissive layers can be formed by, for example, MOCVD.
Next, the emissive layer and the p type GaN layer are etched to expose a portion of the n type GaN layer (step S104). For the etching, in general, a reactive ion etching (RIE) is employed in which an RF is supplied while introducing etching gas to generate plasma for effecting the etching. As the etching gas, Cl-based gas such as BCl3 or a mixture of such gases is used.
After the surface of the n type GaN layer is exposed by etching the p type GaN layer and the emissive layer, a negative electrode is formed on the exposed n type GaN layer (step S105) and a positive electrode is formed on the p type GaN layer (step S106).
As described above, in order to form a device structure such as a light emitting element on an insulative substrate such as a sapphire substrate, a positive electrode and a negative electrode must be formed, and, therefore, a portion of the GaN-based layers must be etched away. Use of a BCl3-containing gas mixture has many advantages over Cl2-based mixtures, such as low etching selectivity between various AlInGaN compounds. BCl3-containing gas mixtures can also be used to etch other materials such as top oxides or thin metal layers, where Cl2-based gas mixtures fail. The use of BCl3-containing gas mixtures is therefore desired for GaN-based device fabrication.
On the other hand, when a gas mixture containing BCl3 is used as the etching gas, there had been a problem in that ohmic contact cannot be easily obtained when the electrode is formed on the n type GaN layer.
FIG. 7 shows current-voltage characteristics for a an example wherein a Ti/Al electrode is formed as the negative electrode on an n type GaN layer which was etched by a gas other than a mixed gas containing BCl3 and for an example wherein a Ti/Al electrode is formed as the negative electrode on an n type GaN layer which was not etched. In FIG. 7, the horizontal axis represents the applied voltage (V) and the vertical axis represents current (mA). For an electrode to be said xe2x80x9cohmicxe2x80x9d, the current-voltage characteristic must be such that the current linearly changes with the change in the applied voltage. In contrast, in the case where the etching was performed using a gas mixture containing BCl31 current did not flow until a voltage of 7 V or greater is applied, that is, the contact is not ohmic and, thus, the light emitting efficiency is decreased.
The present invention was conceived to solve the above problem and one object of the present invention is to realize an ohmic contact between a GaN-based layer and an electrode.
In order to achieve at least the object mentioned above, according to the present invention, there is provided a method for manufacturing a nitride-based compound semiconductor, comprising the steps of (a) forming a p type GaN-based layer and an n type GaN-based layer on a substrate; (b) etching to expose a portion of the GaN-based layer which is not exposed on the front surface among the GaN-based layers; and (c) forming an electrode on the surface of the exposed GaN-based layer. Here, the etching step (b) includes the sub-steps of (b1) exposing a portion of the GaN-based layer through reactive ion etching using gas containing boron, and (b2) removing boron contamination layer on the surface of the GaN-based layer through reactive ion etching using gas which does not contain boron.
When an n type GaN-based layer is formed on the substrate and then a p type GaN-based layer is formed thereon, the n type GaN-based layer is the GaN-based layer which is not exposed at the surface. In such a case, during the etching step, part the layer formed above the n type GaN-based layer is removed to expose the n type GaN-based layer. Use of a BCl3-containing gas mixture has many advantages over Cl2-based mixtures, such as low etching selectivity between various AlInGaN compounds. BCl3-containing gas mixtures can also be used to etch other materials such as top oxides or thin metal layers, where Cl2-based gas mixtures fail. The use BCl3-containing gas mixtures is therefore desired for GaN-based device fabrication. On the other hand, when BCl3 gas is used in the etching step, there is a problem in that the surface of the GaN-based layer is contaminated by boron and ohmic contact cannot be obtained. To this end, in the present invention, the etching step is performed through two separate stages or sub-steps. In the first sub-step, etching is performed using a gas which includes boron (B) such as, for example, BCl3, to expose a portion of the surface of the GaN-based layer while preventing the hardening of the photoresist. In the second sub-step, etching is performed using a gas which does not include boron, such as, for example, Cl2, to remove the B contamination layer on the exposed surface of the GaN-based layer. In this manner, by executing the etching step in two sub-steps, an ohmic contact can be realized without the layer containing B present at the boundary between the GaN-based layer and the electrode.
In one embodiment of the present invention, an n type GaN-based layer, a GaN-based emissive layer, and a p type GaN-based layer are sequentially formed on a substrate, and then the GaN-based emissive layer and the p type GaN-based layer are etched through reactive ion etching to expose a portion of the surface of the n type GaN-based layer. Here, the reactive ion etching step comprises a first sub-step in which BCl3 gas is used and a second sub-step in which Cl2 gas is used. Next, a negative electrode is formed on the surface of the n type GaN-based layer and a positive electrode is formed on the surface of the p type GaN-based layer, to thereby obtain a light emitting element.
FIG. 3 shows a SIMS (secondary ion mass spectrometer) results for a sample in which the GaN-based layers were etched by reactive ion etching using BCl3 gas and then a Ti/Al contact layer was formed. In FIG. 3, the horizontal axis represents time (in seconds) which corresponds to the depth from the surface (Ti/Al surface). It can be seen from FIG. 3 that B is present at the boundary between the Ti/Al which is the electrode material and the GaN-based layer of the semiconductor layer. In other words, when the GaN-based layer is etched using BCl3, its surface is contaminated by B and this contamination layer blocks electrical conduction between the GaN-based layer and the electrode material.
FIG. 4 shows a SIMS result of a sample in which the GaN-based layer was etched through reactive ion etching using BCl3 gas and then etched through reactive ion etching using Cl2, and a Ti/Al contact layer was formed. Upon comparison with FIG. 3, it can be seen that substantially no B is present (B is present only in a negligible amount) at the boundary between the Ti/Al, which is the electrode material, and the GaN-based layer of the semiconductor layer.
According to the present invention, because the GaN-based layer is exposed by the first etching sub-step and the B contamination layer on the surface of the GaN-based layer is removed by the second etching sub-step, the RF power to be applied in the second etching sub-step can be at a level necessary and sufficient for removing only the B contamination layer. Thus, it is possible to use weaker RF power in the second sub-step than in the first sub-step (i.e. RF power of first sub-step greater than RF power of second sub-step). In the second etching sub-step, it is preferable to set the RF power to a degree such that the photoresist which is used for exposing the GaN-based layer is not damaged. Also, because the etching need not be performed beyond the B contamination layer, the depth of the etching is preferably 10 nm or greater, and more preferably 20 nm or greater and 100 nm or less. For example, an etching depth of approximately 50 nm is desirable.